In the past, in ballistic research, it has been often required to observe a projectile immediately after launch from a gun tube, while the projectile is still in the muzzle blast region. X-ray photographs of the projectile in the blast region have permitted measurement of projectile structural integrity and measurement of initial velocity. One of the problems encountered in the past with X-ray photography has been the inability to trigger the X-ray equipment at the proper time to catch the projectile image on the X-ray plate while it is within the blast region.
One of the methods used for making velocity measurements was by use of microwave interferometry. In this method a microwave beam is directed down a gun tube. The microwave signal is reflected back from the projectile as it travels down the tube providing a longitudinal displacement and velocity measurement. The problem with this method was that due to ionized gases in the muzzle blast region, the microwave signal was often severely attenuated, causing signal dropout. At other times, there was sufficient gas blow-by to cause a loss of signal before projectile exit. This method is not suitable as a trigger mechanism.
In other instances high-speed photography has been used for the aforementioned purposes. However, the problem with this method was that in the blast region there is severe attenuation of visible light, thus limiting the view of the projectile and measurement of velocity only to the region beyond the blast region.
X-ray photography provides the best means to observe the projectile and measure the velocity in the blast region. However, the critical problem in the past has been to provide a triggering signal which is precise for different charges and different projectile configurations being tested. Frequently, because of the variable conditions, improper triggering results in the X-ray being triggered at the improper time, and failure to photograph the projectile.
Automatic X-ray trigger may be derived by the use of digital up-down counter circuits which count clock pulses "up" during the time it takes a projectile to pass from one light screen or break wire to another. After the projectile passes the second light screen or break wire, the counter counts "down". When the count reaches zero a pulse is generated to fire the X-ray. Assuming no great change in velocity, the projectile travels approximately the same distance during the count "down" as during the count "up" permitting X-ray heads to be positioned accordingly. The problem with the aforementioned prior art method is that the light screens or break wires must be positioned down range beyond the muzzle blast area. Since any X-ray photographs must be taken beyond the second screen, X-ray photographs of the projectile in the muzzle blast region are not possible.